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They further concluded that the lower CaO branch may represent an olivine control
trend. They observed that K
2
O remains constant within a wide range of MgO
values, and concluded that Ba and Ni plots have the same trends, which demon-
strate that a Ba- and K-rich phase crystallized during the entire period of crystal-
lization. The BaO content increases from
0.5 wt% in phlogopites from primitive
minettes to 4.08 wt% in biotites from more evolved shonkinites. This may indicate
that mica and clinopyroxene fractionation was responsible for maintaining a bulk
D
Ba
value
*
1.0. They suggested that the presence of plagioclase phenocrysts in
these latter rocks indicate that they were derived from different source regions
possibly within the lower crust.
O.Brien and Thirlwall (1988) characterized the source components of the
potassic ma
*
c magmas of the High Wood Mountains province. They further sug-
gested early crystallization of F-poor phlogopite and evidence of explosive erup-
tion, which indicate that the parent magma was characterized by the presence of a
H
2
O
CO
2
-rich
fluid phase, and emplacement of ma
c phonolites was associated
-
with degassing. According to O
c
phonolites and minettes contain mixed phenocrystal assemblages, which may be
due to periodic mixing between the two magmas. The ma
'
Brien et al. the majority of the evolved ma
c phonolites sometimes
contain diopside xenocrysts, occurring at the centre with salite forming the rim. The
phonolites also contain phlogopite (
1 cm long) with resorbed outlines and zoned
olivine (Fo
87
-
90
) xenocrysts. The minettes are constituted of zoned clinopyroxene,
salite and olivine (<Fo
77
or lower) with resorbed phlogopite which are often sur-
rounded by Ti- and Ba-rich biotite.
The primitive minettes are enriched in light REE and Ba (2,000
*
5,000 ppm),
-
Sr (850
gures are
consistent with residual garnet, mica and clinopyroxene in the source. The chem-
istry of olivine, diopside, and phlogopite phenocrysts, in the primitive minettes is
similar to those found in the phlogopite-garnet lherzolite xenolith in the minette
called,
1,200 ppm), Ni (250
330 ppm) and Cr (500
725 ppm). These
-
-
-
Brien et al. suggested that the Highwood
minette magma was produced by partial melting of a phlogopite and garnet-bearing
peridotitic mantle. Chemical analyses of K-rich rocks from Highwood Mountains
are summarized in Table
4.9
.
“
the Thum
”
(Ehrenberg 1982). O
'
4.5.2 The Bearpaw Mountains
The igneous rock complex at the Bearpaw Mountains of northcentral Montana
constitute a near circular outcrop of about 60 km in diameter and has a height of
2,269 m (Macdonald et al. 1992, Fig.
4.12
). The volcanic rocks are observed in a
WSW
ENE-trending anticlinal arch containing Palaeozoic to upper Cretaceous
sedimentary rocks and numerous intrusive bodies. Igneous activity is considered to
have taken place during Eocene (54
-
50 Ma, Marvin et al. 1980).
-
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